Preparation of surfaces for receiving lead sulfide coatings



1366- 1959 M. L. SUGARMAN, JR

PREPARATION OF SURFACES FOR RECEIVING LEAD SULFIDE COATINGS Filed Feb. 16, 1956 LEAD NITRATE PULYl/l/VYL ALCOHOL MIX AGE

IMMERSE (SEED/N6) ALKALINE SOLUTION LEAD-SALT THIOUREA HYDROGEN SULFIDE CLEAN mnmss (MEG/P1771770) 31 MegerLSugm-manJn INVENTOR.

Momm I United States Patr a-t Eastman Kodak Company, Rochester, N-.Y.,.a corporation of New Jersey Application February 16,1956, Serial No. 567,047

12 Claims. (Cl; 117-34) This is a continuation-in-part ofmy applicationSerial No. 276,800 filed March 15, 1952, now abandoned.

This invention relates to photoelectric cells andparticu-larly to the manufacture of infrared sensi'tive lead- 'sulfide cells.

Lead sulfide cells have been prepared in the-past by Chemical deposition or by vacuum evaporation. The present invention belongs in the-chemical deposition class.- Specifically, the present invention has t'o'do with the method of preseeding the surface which isto receive the crystalsof lead sulfide as they are deposited. The present invention is useful asa step in processes of the known type in which lead sulfide is precipitated onto a seeded" glass surface by the addition of thiourea to a highly alkaline solution of a leadsalt. Preseeded surfaces may be used'in various forms of such coating processes but are absolutely essential to the process described by CJQ. Glassey in application'SerialNo. 567,045, filed concurrently herewith, which is a continuation-in-part of application Serial No. 276,799, filed March 15, 1952, now abandoned. The present invention is also applicable to the-improved form of this precipitating process described by Hamrnar and Bennett in application Serial No. 567,046, also filed concurrently herewith, now aban- 'doned, which is a continuation in-part of application Serial No; 276,798, also filed March 15, 1952, now abandoned. The present invention improves the sensitivity of any chemically deposited lead sulfide cell and combines particularly well with the invention ofHammar and Bennett: Altogether cells of.- extraordinarily high sensitivity are consistently produced with. a' sensitivity'over 100 tifnes that usually foundinprior lead sulfidecells and even higher than that found in unusual, accidental, unreproduceable, prior instances.

. The primary purpose of seeding a..glass1or-other surface to receive precipitated crystals :is to \ensurethat the crystal layer is uniform and, in, fact, to ensure-that' the crystals form: preferentially onthe surface itself rather than merely throughout the solution generally. Asidev from the present invention thereare: numerous other ways of producing a preseeded surfaces The seed coat can be produced mechanically or by thermal evaporation or, with some surfaces, merely by dipping the .surfaceinazcationic wettingagent. However, all off these other methods are more orle'ssalike in that they merely encourage preferentialicrystallizationand uniform crystallization on thesurface .in question. They all result in the samersensitivity and-,.in fact, nonesof' them appearstoaffect; thesensitivity; favorably. In fact, most-aredetrimental;to1sensitivity:

Theprocesswaccording to the-present invention, onthe other. hand, does improve the sensitivityxof the final layer and actually :increases the sensitivity by. a factor in excess of two. This is believed to be due to a.different'-form of crystal. That is, it is believed. that! the seed coat according to the present invention starts the ultimate lead sulfide crystal forming with a different crystal structure than that provided by other seed coats and that the 2 ultimate crystal has a different nature which accounts for. the increased sensitivity.

According'to'the present invention a glass plate is cleaned thoroughly and then isbrought into contact with a special colloidal solution either by dipping in the solution or having the solution poured over the glass plate and the plate is allowed to remain in contact with this solution for a period between /2- and 26 hours, usuallyfor about 2 hours or more and until the slide is requiredfor use in the lead sulfide precipitation process itself. The seeding solution is a colloidal lead sulfide sol made by adding to an aqueous solution of a water soluble lead salt (such as lead nitrate, lead acetate, lead chlorate or lead perchlorate) and a stabilizer (such as poly-vinyl alcohol) a solution of hydrogen sulfide in which the amount of hydrogen sulfide is more'than that necessary to convert all of the lead salt to lead sulfide. However, the excess of hydrogen sulfide should not be too-great. Specifically, it has been found advantageous not to have much more than ten times the minimum amount of hydrogen sulfide. In practice about five times the minimum amount worksquite well. The reason for not=liaving too great a concentration of hydrogen sulfide is' 'th'atthe latter itself tends to provide a seed coat and hence there would be two kinds of seed coats on the glass surface, one of which provides the high sensitivity of'the presentinvention and the other of which, being due to the hydrogen sulfide alone, tends to'dilute the effect of the present invention and to result in crystals of lower sensitivity. Before the solution is brought into contact with the clean glass plate, it should be allowed to age for at least 30 minutes, preferably for about an hour.

The preferable concentration of lead sulfide in the seeding solution is between 1' partin 5,000 and 1 part in 100,000. The total sulfide concentration including the lead sulfide and the-excess hydrogen sulfide is preferably about .002 N. Other stabilizers such as gelatin, starch, various gums such as gum tragacanth, methyl cellulose or carboxy methyl cellulose are useful but polyvinyl alcohol acts both as a wetting agent to improve uniformity over the cell areaand as acolloid stabilizer. It canbe used successfully in relatively low concentrations. In vthe aqueous solution of lead nitrate the concentration of polyvinyl alcohol is between .01 percent and .5 percent.

Thus the methodof forming a photosensitive surfacexof lead sulfide crystals according to the present invention consists of seeding'a clean glass plate as-outlined. above and then immersingthe seeded'plate into a highly alkaline lead salt solution to which thiourea has been added to precipitate; lead sulfide onto the plate.- The' plate is then merely dried, anyloose crystals being cleaned otfi necessary, and the plate is ready for use. I

The accompanying drawing is'a flow chart illustrating schematically a preferred embodiment of the'present invention.

In the drawing the various solutions are, for convenience, merelyindicated as being in test tubes and the careful handling of the glass plate is represented by tongs.

The seeding solution is prepared by the addition of a solution 10containing225 ml. of water, 25 ml. of .5 percent solution of polyvinyl alcohol and .035 gram of CP lead' nitrate- After thesalt has dissolved, a sufficieht volume of, stock hydrogensulfide solution 11 ofknown strength is; added to result in a finalv sulfideconcentration in -solution 12-. of .002 N. In practice a sample-of the solution 1'1 may be titrated, for example, against a potassium iodate solution and the amount then calculated to'give the; desiredmoncentration in the solution 12 or a sampleofthesolution 12 itself may be titrated to test the final concentration. The solution 12 is a clear deep red and is aged for one hour resulting in the colloidal solution 13. At that time it is brought into contact with a clean glass plate 20 either by immersing the plate in the solution 13 as shown in the drawing or by pouring the solution over the glass plate or a row of such plates mounted in a suitable rack. The seeding is allowed to continue for 2 hours or more and results in a seeded glass plate 25 which is then ready to receive a lead sulfide precipitate by standard procedure or preferably by the Hammar and Bennett procedure discussed above. The glass plate 20 as shown toward the upper right of the drawing is first cleaned free of fingerprints and other grease by being immersed in a boiling solution 21 of sodium dichromate, sulfuric acid and water. Alternatively and quite satisfactorily, clean glass blanks can be purchased with a quality that requires no further cleaning. The clean plate is then rinsed in tap water, preferably followed by rinsing in distilled water, dried in air or blotted, and is then immersed in the seeding solution or placed in a rack to receive the seeding solution.

The seeded glass plate, as shown toward the lower right of the drawing is immersed in an alkaline solution 26 of lead salt (such as lead nitrate, lead acetate, lead chlorate or lead perchlorate) to which a thiourea solution has been added. The glass plate 25 is preferably placed at an angle so that one surface receives precipitated crystals unaided by gravity and uncontaminated by the gross precipitate which tends to settle to the bottom of the solution. The uncontaminated surface 30 is much more sensitive than the surface 31 which during precipitation had been facing at least partly upward. According to the Hammar and Bennett procedure mentioned above, the lead salt solution 26 also contains a specified small proportion of a copper salt and may also contain some sodium sulfite. According to the Glassey inven- 'tion also mentioned above, there is preferably a specified delay between the mixing of thiourea and lead salt solution 26 and the immersion of the preseeded plate 25.

Having thus described the preferred embodiment of my invention I wish to point out that the invention is limited only by the scope of the appended claims.

I claim:

l. The method of seeding a surface of a cleanglass plate for receiving highly sensitive, precipitated, lead sulfide crystals which comprises adding to an aqueous solution of a soluble lead salt selected from the group consisting of lead nitrate, lead acetate, lead chlorate and lead perchlorate and a colloid stabilizer, hydrogen sulfide solution in an amount between that necessary to convert all of the lead salt to lead sulfide and ten times as much, allowing the solution to age for at least 30 minutes and then bringing the solution and said surface into contact for a period between /2 and 36 hours.

2. The method according to claim 1 in which the solutions are combined in concentrations which give a lead sulfide concentration between 1:5,000 and 1:lO0,000 parts.

3. The method according to claim 1 in which the solutions are combined in concentrations which give a total sulfide concentration about .002 N.

4. The method according to claim 1 in which the stabilizer is polyvinyl alcohol in a concentration in the aqueous solution between .01 percent and .5 percent.

5. The method of seeding a surface of a clean glass plate for receiving highly sensitive, precipitated, lead sulfide crystals which comprises adding to an aqueous solution of lead nitrate and polyvinyl alcohol, hydrogen sulfide solution in excess of that required to convert all of the lead nitrate tolead sulfide, the lead nitrate and hydrogen sulfide concentrations being those which give a total sulfide concentration about .002 N and the polyvinyl alcohol concentration in the aqueous solution being between .01 percent and .5 percent, allowing the solution to age for at least 30 minutes and then bringing the solu- 4 tion and said surface into contact for a period between A and 36 hours.

6. The method of forming a photosensitive layer of predominantly lead sulfide crystals which comprises seeding a surface of a clean glass plate by adding to an aqueous solution of a soluble lead salt selected from the group consisting of lead nitrate, lead acetate, lead chlorate and lead' perchlorate, and a colloid stabilizer, hydrogen sulfide solution in an amount between that necessary to convert all of the lead salt to lead sulfide and ten times as much, allowing the solution to age for at least 30 minutes and then bringing the solution and said surface into contact for a period between /2 and 36 hours, and then immersing the seeded plate into a highly alkaline solution of a water soluble lead salt selected from the group consisting of lead nitrate, lead acetate, lead chlorate and lead perchlorate, to which thiourea has been added to precipitate lead sulfide onto the plate and drying the plate.

7. The method of forming a photosensitive layer of predominantly lead sulfide crystals which comprises seeding a surface of a clean glass plate by adding to an aqueous solution of lead nitrate and polyvinyl alcohol, hydrogen sulfide solution in excess of that required to convert all of the'lead nitrate to lead sulfide, the lead nitrate and hydrogen sulfide concentrations being those which give a total sulfide concentration about .002 N and the polyvinyl alcohol concentration in the aqueous solution being between .01 and .5 percent, allowing the solution to age for at least 30 minutes and then bringing the solution and said surface into contact for a period between A and 36 hours, and then immersing the seeded plate into a highly alkaline solution of a water soluble lead salt selected from the group consisting of lead nitrate, lead acetate, lead chlorate and lead perchlorate, to which thiourea has been added to precipitate lead sulfide onto the plate and drying the plate.

8. The method of seeding a surface of a clean glass plate for receiving highly sensitive, precipitated, lead sulfide crystals which comprises adding to an aqueous solution of a soluble leadnitrate and polyvinyl alcohol, hydrogen sulfide solution in an amount between that necessary to convert all of the lead nitrate to lead sulfide and ten times as much, allowing the solution to age for at least 30 minutes and then bringing the solution and said surface into contact for a period between to and 36 hours.

9. The method according to claim 8 in which the solutions are combined in concentrations which give a lead sulfide concentration between 1:5.000 and 1:100,000 parts.

10. The method according to claim 8 in which the solutions are combined in concentrations which give a total sulfide concentration about .002 N.

11. The method according to claim 8 in which the concentration of the polyvinyl alcohol is between .01 and .5 percent.

12. The method of forming a photosensitive layer of predominantly lead sulfide crystals which comprises seeding a surface of a clean glass plate by adding to an aqueous solution of lead nitrate and polyvinyl alcohol, hydrogen sulfide solution in an amount between that necessary to convert all of the lead nitrate to lead sulfide and ten times as much, allowing the solution to age for at least 30 minutes and then bringing the solution and said surface into contact for a period between 36 and 36 hours, and then immersing the seeded plate into a highly alkaline solution of a water soluble lead salt selected from the group consisting of lead nitrate, lead acetate, lead chlorate and lead perchlorate to which thiourea has been added to precipitate lead sulfide onto the plate and drying the plate.

No references cited. 

12. THE METHOD OF FORMING A PHOTOSENSITIVE LAYER OF PREDOMINANTLY LEAS SULFIDE CRYSTALS WHICH COMPRISES SEEDING A SURFACE OF A CLEAN GLASS PLATE BY ADDING TO AN AQUEOUS SOLUTION OF LEAS NITRATE AND POLYVINYL ALCOHOL, HYDROGEN SULFIDE SOLUTION IN AN AMOUNT BETWEEN THAT NECESSARY TO CONVERT ALL OF THE LEAD NITRATE TO LEAD SULFIDE AND TEN TIMES AS MUCH, ALLOWING THE SOLUTION TO AGE FOR AT LEAST 30 MINUTES AND THEN BRINGING THE SOLUTION OF SAID SURFACE INTO CONTACT FOR A PERIOD BETWEEN 1/2 AND 36 HOURS, AND THEM IMMERSING THE SEEDED PLATE INTO A HIGHLY ALKALINE SOLUTION OF WATER SOLUBLE LEAD SALT SELECTED FROM THE GROUP CONSISTING OF LEAD NITRATE, LEAD ACETATE, LEAD CHLORATE AND LEAD PERCHLORATE TO WHICH THIOUREA HAS BEEN ADDED TO PRECIPITATE LEAD SULFIDE ONTO THE PLATE AND DRYING THE PLATE. 